1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, and more particularly to an image forming apparatus such as an inkjet printer, based on a dot recording method, and technology for efficiently transferring print data used in such an image forming apparatus.
2. Description of the Related Art
In the technical field of inkjet printers, a so-called matrix type of inkjet head is known in which nozzles for ejecting ink droplets are arranged in a two-dimensional array. In the matrix type head, due to the arrangement of the pressure chambers and the ink flow channels, it is necessary for the pitch between the nozzles to be larger than the pitch between pixels, and therefore, the adjacency relationship of the nozzles which are adjacent in the head cannot be directly made correspondent to the adjacency relationship of the pixels of the printed dots which are adjacent on the recording medium (on the image recorded on the recording paper). Therefore, an oblique matrix arrangement structure is adopted in which the positions of the nozzles are arranged in a slightly staggered fashion (see FIG. 2).
When a prescribed image (for example, the bitmap image shown in FIG. 6) is formed using the matrix type head of this kind, print data generated previously in a memory is read out and transferred to the matrix type head, and the head is driven so as to eject ink droplets from the corresponding nozzles of the head. However, in this case, there are the following problems.
(1) The print data for nozzles which are adjacent on a straight lute in the main scanning direction of the head is arranged in discrete positions in the image memory (see FIG. 6).
(2) In particular, in an actual matrix type head, the nozzles are formed at distances of several ten pixels to owe hundred pixels apart, and the overall width of the head (the dimension in the paper conveyance direction) is no less than one thousand lines (one thousand pixels). Consequently, for each droplet ejection operation, print data for all of the nozzles of the head (data for one droplet ejection operation) must be gathered from the broad image region covered by the head (for instance, a range 170 defined with the dotted lines in FIG. 6). A large-capacity memory, such as an image page memory, is generally constituted of a dynamic RAM (DRAM), but a feature of a DRAM is that access to non-consecutive addresses of this kind is slow (a fraction of the speed of access to consecutive addresses).
(3) The image page memory is usually build on word units (8-bit, 16-bit, 32-bit, . . . ), which means that read-out is performed in word units. However, since the print data is 1 to 2 bits per nozzle (in many cases, the inkjet printer has around 1 to 3 droplet ejection modification levels) and the data for adjacent nozzles is not included in the same word as described above, then it is necessary to read out a whole word in order to read out 1 bit, and to discard the remaining data of the word. This means that the memory reading efficiency is poor.
(4) The facts that the memory read addresses are distributed discretely (random reading) and then the reading efficiency is poor are not major problems if the printer speed is slow (the head has a small number of nozzles and a low drive frequency). However in compositions which print at high speed, such as a full line-type of head having a full page width, where the apparatus is required to have high-speed performance, the speed of memory read-out and data transfer is a barrier to achieving this high-speed performance.
In response to these problems, Japanese Patent Application Publication No. 2008-183884 discloses technology including a line memory which stores data for one line from amongst print data for one page, a rearrangement device which changes an arrangement sequence of the data for one line stored in the line memory, and an image buffer memory which stores print data that has been rearranged by the rearrangement device, wherein the rearrangement device has a composition for rearranging data in such a manner that data for pixels corresponding to recording elements of the recording head that are adjacent in an alignment on a straight line following a direction perpendicular to a relative movement direction of the recording medium is arranged in the same word or in adjacent words, and the image buffer memory has a storage capacity for storing the rearranged print data for an image region which corresponds at least to a surface area occupied by a two-dimensional matrix arrangement of the recording elements in the recording head. According to this technology, it is possible to read out print data efficiently from consecutive addresses in an image buffer memory, and therefore high-speed read-out and high-speed transfer become possible. Furthermore, since the memory can be composed readily using an inexpensive DRAM, then it is possible to achieve reduced costs.